1. Field of the Invention
The present invention relates to a speed-change control system for an automatic transmission including a vehicle speed sensor for electrically detecting vehicle speeds, an engine load sensor for detecting engine loads and an electronic circuit to which are supplied outputs from the sensors and comparing these outputs with predetermined relations between these outputs to produce signals for instructing speed changes.
2. Description of the Prior Art
An automatic transmission of this kind as shown in FIG. 1 has been suggested, wherein a vehicle speed sensor 1 detects revolutions of an output shaft of the transmission to produce actual vehicle speed signals which are proportional to actual vehicle travelling speeds.
An engine load sensor 2 detects loads of an engine to generate signals corresponding to the loads. In effect, the sensor 2 may be a conventional throttle opening sensor for generating signals proportional to opened degrees of a throttle of the engine or a conventional negative pressure sensor for generating signals proportional to negative pressures in an intake duct of the engine, which may be selected as the case may be. In the following explanation with respect to the prior art and embodiments of the invention, the throttle opening sensor is used as the engine load sensor.
Speed change program memory circuits 3 and 4 are supplied with the engine load signals from the engine load sensor 2 and produce reference change speed signals representing vehicle speeds which are suitable for the engine loads by means of a predetermined engine load-speed change reference program. The speed change program memory circuits 3 and 4 store the engine load-speed change reference programs as shown in FIG. 2, respectively.
With the program shown in FIG. 2 illustrating an example of an automatic transmission having three speed positions, the graph includes 2 to 1 down-shift program 21, 1 to 2 up-shift program 22, 3 to 2 down-shift program 23 and 2 to 3 up-shift program 24.
As shown in FIG. 2, for example as to 2 to 1 down-shift program 21 and 1 to 2 up-shift program 22, reference change speeds V.sub.D and V.sub.U are determined for the same throttle opening .theta..sub.1, respectively. The reference up-shift change speed V.sub.U is set on the side higher than the reference down-shift change speed V.sub.D. The difference between the reference speeds V.sub.U and V.sub.D serves as a hysteresis to obtain a stable speed change. The same holds true in case of the 3 to 2 down-shift program 23 and the 2 to 3 up-shift program 24.
The speed change memory circuits 3 and 4 are supplied with program select signals later described to select associated programs in first, second and third speed ranges as shown in FIGS. 3a-3c among the stored speed change programs. FIGS. 3a-3c illustrate the selected programs when the transmission is in first, second and third speed positions, respectively. When the transmission is in the second speed position in FIG. 3b, the 2 to 1 down-shift program 21 and the 2 to 3 up-shift program 24 are alternately selected by a control of time division by means of clock pulses from a pulse generator 10. (These operations will be explained later).
The speed change program memory circuits 3 and 4 store respective different programs. The memory circuit 3 stores a speed change program which produces lower reference change speed signals for the same throttle opening suitable for travelling on a flat road and the memory circuit 4 stored a speed change program which produces higher reference change speed signals for the same throttle opening suitable for an uphill road.
A multiplexer 5 selects any one of reference change speed signals received from the speed change program memory circuits 3 and 4 by means of an inclined angle sensor 6 or a program change-over switch 7 later described, which selected reference change speed signal is then supplied to a comparator 8.
The inclined angle sensor 6 produces signals proportional to inclined angles of roads in a travelling direction of the vehicle. If the signal exceeds a determined value, the reference change speed signal from the speed change program memory circuit 4 is supplied through the multiplexer 5 to the comparator 8. The program change-over switch 7 is manually operated by a driver to generate a signal which is applied to the multiplexer 5 for selecting the speed change program memory circuit 3 or 4.
The comparator 8 compares the signal proportional to an actual vehicle speed from the vehicle speed sensor 1 and the reference change speed signal received through the multiplexer 5 to produce a high level signal H when the actual vehicle speed signal is greater than the reference change speed signal and to produce a low level signal L when the actual vehicle speed signal is smaller.
A speed change control circuit 9 receives the output signals H and L of the comparator 8 to control an energization and deenergization of a plurality of solenoid valves for controlling operations of a clutch and a brake which serve to change over connected relations of a sun gear, a carrier and a ring gear of a planetary gear unit to determined speed positions of the transmission. Furthermore, the speed change control circuit 9 supplies the program select signals to the speed change program memory circuits 3 and 4 to select the relevant speed change program as above described referring to FIGS. 3a-3c.
When the transmission is in the second speed position, the speed change control circuit 9 alternately produces program select signals for selecting the 2 to 1 down-shift speed change program 21 and the 2 to 3 up-shift speed change program 24. The transmission is kept in the second speed position, if the output of the comparator 8 is the H signal when the program select signal for selecting the speed change program 21 is produced and if the output of the comparator 8 is the L signal when the program select signal for selecting the speed change program 24 is produced. If the output of the comparator 8 does not correspond to the above program select signals, the speed change control circuit 9 operates to change the solenoid to be energized for changing the transmission into the first or third speed position, with the result that the speed change control circuit 9 produces the program select signal for selecting the speed change program 22 or 23 as shown in FIGS. 3a or 3c.
With this arrangement of the prior art, the speed change program corresponding to a speed position is selected by the program select signal and the signal from the throttle opening sensor 2 corresponding to an engine load determines a reference change speed with which an actual vehicle speed sensed in the vehicle speed sensor 1 is compared to select the up-shift or down-shift for a suitable speed change.
With this arrangement, however, a driver must select the program memory circuit 3 or 4 in consideration to a grade of a road by a manual operation of the program change-over switch 7 which is troublesome. On the other hand, the inclined angle sensor 6 does not detect the change in weight of load on the vehicle. If the speed change program memory circuit 4 is set to store a speed change program the most suitable for a heavy load on an uphill road, the speed change program memory circuit 4 is selected even when the vehicle is under a slightly loaded condition, so that the up-shift of the transmission is not readily effected.